Stabilizers for synthetic resinous materials are routinely stated to be useful in conjunction with phenolic antioxidants, pigments, colorants or dyes, secondary stabilizers which may be hindered amines, metal deactivators, etc. yet fail to recognize the problems of making such combinations. For example, phenolic antioxidants have a high proclivity to turn yellow; the same pigment functions unpredictably in different substrates; some pigments are prodegradants in polyolefins but not in polyacetals; and hindered amine light stabilizers are generally unable to stabilize color due to a pigment over the life of a polyolefin exposed to bright sunshine.
The continued improvement in stability provided by improved hindered amine light stabilizers allow a wide variety of pigmented thermoformed shaped articles including molded articles, fibers and sheets of stabilized polyolefins, and other normally solid predominantly ethylene-and propylene-containing copolymers (together referred to hereafter as "PO" for brevity) to have a useful life longer than the period over which the original color survives. Nevertheless the consumption of such articles has grown apace. Among the most popular are phthalocyanine-pigmented articles having a well-known proclivity to change color long before they lose their integrity. This is particularly unfortunate because phthalocyanine pigments in polyethylene (PE) and polypropylene (PP) are ideal pigments in most other respects. Retention of blue or green color, and other shades derived from phthalocyanine pigments in articles exposed to sunlight over their useful life, is of great practical value in automotive fabrics, molded PO household goods, and clothing made from woven or non-woven fabrics of the pigmented PO fibers or sheet. To combat the problem of color fading, a better solution is constantly being sought to decelerate color loss which is at least as important as stabilization of the mass of the PO.
This invention particularly relates to the stabilization of shaped articles of PO, specifically of PE and PP articles colored with phthalocyanine pigments which provide colors across the entire visible spectrum; more particularly, it relates to those pigments which provide either a blue or a green color, and various shades thereof.
It is known that several stabilizers, particularly hindered amine stabilizers ("HALS"), by themselves, provide excellent stabilization of PO to heat, light and ultraviolet radiation; and, some hindered phenol stabilizers are antioxidants which provide both excellent thermal oxidative stabilization, and light stabilization of PO, but such stabilization does not extend to that of color in phthalocyanine-pigmented PO. For example, the combination of a hindered piperidyl compound such as commercially available Chimassorb 944 has been combined with a benzoate type stabilizer such as 3,5-disubstituted-4-hydroxybenzoate, and a phosphite such as bis-di-t-butyl-pentaerythritol phosphite commercially available as Ultranox (see Japanese publication JP-230401 (1987) to Sumitomo Chem Ind KK). But there is no indication that such a combination might have been notably effective to stabilize any pigment or dye.
In like vein, U.S. Pat. No. 4,753,979 to Conetta et al disclose that a single polysubstituted piperazinone moiety (PSP) connected to alkyl, alkenyl, aralkyl, hydroxyalkyl, alkanoyloxyalkyl, alkenoyloxyalkyl and bezoyloxyalkyl; or two PSPs connected by an alkylene bridge, may be combined with a host of other stabilizers, including numerous 3,5-di-t-butyl-4-hydroxybenzylphosphonates. But metal complexes of the phosphonates appear either to have been overlooked, or deemed unlikely to be of any significant benefit. Of course, it would not have been possible to foresee that connecting at least three or two PSPs to one, or two triazine rings, respectively, in the same molecule, might provide a unique benefit relative to limiting the loss of color due to phthalocyanine pigments in PO to .+-.2 units.
More recently, U.S. Pat. No. 4,816,507 to Cantatore et al discloses that certain polyalkyleneamines with pendant polysubstituted piperidyl groups having a methylated N atom are effective light stabilizers in combination with all nickel compounds including nickel salts of 4-hydroxy3,5-di-tert-butylbenzylphosphonic acid monoalkyl esters (3,5-DHBP), all fillers and reinforcing agents, and all other additives, including pigments. But there is no indication what effect such all-inclusive combinations might have on the color stability of pigments of any kind.
Of specific interest is that numerous pigments for PO, by themselves, provide a significiant level of stabilization to PO, but there are many which have no noticeable effect on stability, and still others which accelerate degradation, that is, are prodegradants. To date, the only reliable method of determining to which group a pigment belongs, is by actually testing it in a particular substrate of interest. It is known, for example, a pigment which is a stabilizer in PP may be a prodegradant in a polyacetal.
Phthalocyanine pigments are known to have good color stability. They also provide some measure of light stabilization by virtue of their ability to block the path of radiation, thus shielding phthalocyanine-pigmented polymer. Such small measure of light stabilization is observed in PP at about 0.4 phr. However, in combination with a stabilizing amount of a known HALS primary stabilizer, stabilization provided by 0.4 phr of a phthalocyanine pigment is not substantial. But increasing the concentration of pigment above about 1 phr may produce "bronzing". "Bronzing" is basically a surface effect which is evident to the naked eye when the surface of the pigmented article is viewed from different angles. Excessive migration of the pigment to the surface results in an apparent metallic sheen or glaze. Since the phthalocyanine pigment is used only for tinctorial purposes in PO, the concentration of pigment is limited to a level insufficient to cause bronzing, usually about 1 phr.
Phthalocyanine pigments have a generally flat tetra benzo tetra azo porphin structure. The pigments are usually made by the reaction of a phthalic acid derivative at a temperature of about 190.degree. C. with a source of nitrogen such as urea and a metal or metal salt. Molybdates, vanadates, and certain compounds of titanium have been found to be useful catalysts for this condensation reaction. Phthalocyanine pigments available as commercial products are (a) Blue 15, Blue 16 and Blue 29, with shades of blue available, for example, as Blue 15:1 through Blue 15:6; and (b) Green 7, Green 36-3Y, Green 36-6Y.
Commercially available PO articles, and fibers in particular, have successfully been stabilized against ultraviolet (UV) light degradation and have good resilience and heat stability with a wide spectrum of HALS. But such stabilized PO articles have poor dyeability because PO is essentially unreactive with most dyes. This poor dyeability of PO dictates that the PO articles be pigmented for long-term stability of PO if it is to be colored with many popular colors. With particular respect to PP articles pigmented with blue, green and shades thereof, which pigmented articles are in high demand, the pigments most often used are Blue 15 and Green 7 respectively. The problem is that the use of such pigments in combination with known stabilizers, both hastens the degradation of the PO articles when exposed to sunlight, and accelerates the fading of their color over time. When combined, stabilizers and phthalocyanine pigment, are subjected to a two-pronged attack on the longevity of phthalocyanine-pigmented articles in normal use, thus vitiating their marketability
Fabrics made from phthalocyanine-pigmented PO, and especially PP fibers, are highly popular in automobiles, boats, outdoor clothing, and other such uses where the fibers degrade at such an unacceptably high rate upon exposure to sunlight, that they are soon transformed into nonuniformly colored articles sporting a wide spectrum of unwanted shades of blue, green, pink and orange. The obvious way to cope with this color degradation problem is to use far more pigment than is required to provide the desired color, with the expectation that, upon suffering the expected color degradation, the coloration of the remaining non-degraded pigment will maintain acceptable, if not the original, color. Except that `loading up` the HALS-stabilized PO article with excess phthalocyanine pigment to maintain tinctorial strength, simply accelerates degradation of the PO fibers because the pigment has a high proclivity towards reaction with commonly used HALS, and other additives such as antioxidants and antiozonants, used to provide melt-stability to the PO.
Typically, several additives are combined in PO before it is melt-extruded into fiber, each additive specifically designed to provide a different zone of stabilization, the main zones being (a) melt extrusion stability, (b) long term thermal oxidative stability during conditions expected to be encountered during use, (c) UV light stability in bright, direct sunlight, and by no means of least importance, (d) stable tinctorial strength to maintain the desired color. Combining several additives known to be effective for each specific purpose, in PO articles generally, and fibers in particular, is likely not to produce the desired results because of objectionable side effects due to interaction between the additives.
For example, thiodipropionate compounds such as dilauryl (DLTDP) and distearyl (DSTDP) help control meltstability despite an odor problem, and certain phosphites control melt flow while depressing the tendency of PO fibers to `yellow` because the fibers usually contain a hindered phenol antioxidant. The hindered phenol antioxidant increases long term stability but accelerates yellowing. It is known that a hindered phenol antioxidant and a thiodipropionate are most effective when used together in PO. Certain HALS provide not only excellent UV stability but also such good long term thermal stability that the PO articles will outlast some of the pigments used to color the articles, but when combined, the interaction of stabilizers and pigment is unpredictable. Therefore various combinations of stabilizers have been used, the HALS being combined with hindered phenol antioxidants, phosphites and thiodipropionate, until by extensive trial and error a suitable combination is found.
Pigments are selected with an eye to their effect on the processing of the PO articles, the stability requirements of the end product, the pigment's interaction with the other additives to be used, the color requirements, and the cost of producing the pigmented PO articles. The intense thrust towards using inexpensive PO sheet, fibers and assorted molded articles in the automobile industry where the colors blue and green are in high demand decreed that, despite their high cost, phthalocyanine pigments be used, because of their intense tinctorial strength and color stability; and, that phthalocyanine pigments be combined with a compatible UV stabilizer. It was found that the most damaging factor in the stability of phthalocyanine-pigmented PO fibers was their interaction with the hindered amine UV stabilizers used.
The commercial use of pigments in PO articles, particularly fibers, requires that the color stability of the PO fiber be such that it equals the useful life of a fabric or other article produced from the PO, when the article is exposed to heat and light. Because the stabilizers used generally affect color, though they are not regarded as colorants, and pigments may affect thermal and UV light stability even if they are not known to have such activity, one cannot estimate what the net effect of the interactions might be. (see "Influence of Pigments on the Light Stability of Polymers: A Critical Review" by Peter P Klemchuk, Polymer Photochemistry 3 pg 1-27, 1983).
We continued our tests with numerous combinations of stabilizers in Blue 15-pigmented fibers, screening the samples to determine whether an unacceptable level of color loss was obtained before the fibers disintegrated. We measured the degree of degradation of the pigmented fibers both by visual observation, and by "scratch testing" (described herebelow) the surfaces of exposed fibers.
Fiber degradation is a phenomenon which is easily visible to the naked eye upon inspection of a degrading pigmented yarn exposed either in a Weather-O-Meter in presence of moisture, or, to bright sun (tests are conducted in the Florida sun) under ambient conditions of humidity. Unstabilized Blue 15-pigmented PP fibers exposed to the Florida sun show no fading because the pigmented fibers degrade far more rapidly than the pigment, which results in continual sloughing off of layers of fiber exposing bright undegraded pigment. Degradation of stabilized PP fibers is characterized (i) by a fuzzy, peach-skin-like appearance of the surface of the fabric (made with the pigmented fibers), and (ii) the problem of fading color.
Of particular interest is the peculiar UV-stabilization effect of an oxo-piperazinyl triazine stabilizer ("PIP-T" for brevity) in which all substitutable positions on each triazine ring are polysubstituted piperazin-2-ones. Such PIP-T HALS are disclosed in U.S. Pat. No. 4,480,092 to John T. Lai et al, for their UV-light stabilization in PP, and, because of the presence of the polysubstituted piperazinone (PSP) group in large PIP-T molecules, were routinely tested in PP plaques for such stabilization-effectiveness as each PIP-T might have. The majority of PO now pigmented with phthalocyanine pigments is now used in PO fibers. Because of the known generally higher sensitivity of pigmented fibers to degradation, compared to plaques, most testing for stabilization was done with fibers. It was during such testing that the remarkable effectiveness of a combination of two known primary stabilizers with phthalocyanine-pigmented PO fibers was noted.
By a "primary" stabilizer we refer to one which provides either long term thermal oxidative stability during conditions expected to be encountered during use, or, UV light stability in bright, direct sunlight. Melt extrusion stability to stabilize the PO during processing is typically provided by secondary stabilizers. Though the present invention does not require the use of any secondary stabilizer, in those instances, for example in fiberspinning, where the melt is extruded at about 270.degree. C. (for PP fibers) more than once to obtain better pigment distribution, the PO may contain a small amount, less than 0.1 phr of a melt (or "process") stabilizer.
As one would expect, some pigments enhance heat and light stability of PO articles stabilized with a particular antioxidant and hindered amine stabilizer. Other pigments have the opposite effect with the same combination. Until tested, one cannot predict with reasonable certainty, what the effect will be. For example, with a nickel-containing stabilizer, Red 101 (iron oxide) is a prodegradant. With the more effective hindered amine stabilizers, both Yellow 93 and Red 144 are prodegradants. The effect of these pigments in stabilized PO articles could not have been predicted by their behavior in unstabilized pigmented fibers, or by their behavior with a different stabilizer. With a nickel-containing stabilizer, Blue 16 is a stabilizer (not a prodegradant), but Blue 16 is a prodegradant with Tinuvin 770 in the GM Weather-O-Meter test (details of which are provided herebelow). Yellow 93, a stabilizer when no other stabilizer is present, is neutral with nickel stabilization but is a prodegradant with Tinuvin 770 (see "Stabilization of Polypropylene Fibers" by Marvin Wishman of Phillips Fibers Corporation in a paper presented at the 1985 International Conference on "Advances in Stabilization and Controlled Degradation of Polymers" in Lucerne, Switzerland). Specifically with respect to blue PP fibers, the problem was to find a combination of stabilizers which circumvented the proclivity of Blue 16 to degrade the PP fibers when the pigment is combined with a conventional AO and UV- light stabilizer. Because Blue 16 was a prodegradant it seemed desirable to use only as much of it as would provide the desired tinctorial effect for the required period of time, namely the useful life of the stabilized PP fiber.
The effect of a large number of pigments on the stability of PP fibers stabilized with Tinuvin 770 has been reported by Steinlin and Saar (see "Influence of Pigments on the Degradation of Polypropylene Fibers on Exposure to Light and Weather", paper presented at the 19th International Manmade Fiber Conference, Sept. 1980 in Austria).
In the same vein, like other workers before us, we tested a large number of combinations of primary stabilizers with Blue 15, and tested them mainly in PP. We opted to use large stabilizer molecules, comparable in size to the size of phthalocyanine, and confirmed that Chimassorb 944 in combination with Blue 15, stabilizes PP fiber but does not stabilize the blue color. Chimassorb 944 is a HALS molecule of comparable size to that of PIP-T, and like PIP-T is a hybrid molecule containing a hindered cyclic amine (piperidine) and a triazine ring. Chimassorb 944 is an oligomer in which the repeating unit combines a hexamethylene diamine having polysubstituted piperidyl substituents on the N atoms, the substituted diamine unit being connected to a triazine ring in which one of the other substituents is a branched chain alkylamine, and the last substituent is also a hexamethylene diamine unit.
Mainly because Chimassorb 944 contains pendant piperidyl rings rather than piperazinone rings, but perhaps also because of the relatively elongated structural configuration of the oligomeric molecule, we found that Chimassorb 944, used in combination with a 2:1 complex of an alkylated phenol phosphonate with a Group VIII or Group IIA metal, e.g. a metal bis[0-alkyl(3,5-di-t-butyl-4-hydroxybenzyl)] phosphonate ("3,5-DHBP"), is not as effective with phthalocyanine-pigmented PP as the combination of the 3,5-DHBP with a PIP-T. The unexpected and particularly noteworthy boost of color-stability derived from a 3,5-DHBP is thought to be due to the electron-withdrawing effect of the para- position of the alkylated phenol phosphonate substituent.
Though PIP-T, referred to in the aforementioned '092 Lai patent was known to be an excellent UV stabilizer in colorless organic materials when used in combination with antioxidants, there was nothing to suggest that the size and structure of the PIP-T molecule in combination with a 3,5-DHBP might be uniquely effective to stabilize phthalocyanine-pigmented PE and PP articles, alone among other polymers tested.